Lithium ion secondary batteries are broadly used as energy devices with a high energy density as power sources for portable information terminals such as notebook computers, cell phones, and PDAs (Personal Digital Assistant). For typical lithium ion secondary batteries, a group of wound electrodes obtained by layering a positive electrode, an insulation layer, a negative electrode, and an insulation layer in this order and coiling the same, or a group of laminated electrodes obtained by layering a positive electrode, an insulation layer, and a negative electrode, has been used. As an active material for a negative electrode, a carbon material having a multilayer structure capable of intercalating a lithium ion between layers (i.e., forming a lithium intercalation compound) and releasing the same is mainly used. As an active material for a positive electrode, a lithium-containing metal complex oxide is mainly used. As an insulation layer, a polyolefin porous film is mainly used. Such lithium ion secondary batteries have high battery capacities and output powers as well as superior charge and discharge cycle performances.
Although lithium ion secondary batteries are at a high level in terms of safety, further improvement of safety has been demanded in view of their high capacities and high power. For example, when lithium ion secondary batteries are overcharged, there is possibility of heat generation. Further, heat may also be generated by occurrence of an internal short-circuit. Moreover, since lithium ion secondary batteries contain a nonaqueous electrolyte containing an organic solvent, there is the possibility that the generated heat will cause chemical degradation of the organic solvent to generate a gas, leading to a trouble such as an increase in an internal pressure of a battery or the like.
Currently, further improvement of the safety of lithium ion secondary batteries is intended by cutting off a current inside the batteries to suppress heat generation, when the lithium ion secondary batteries are overcharged or the like. Examples of means for improving safety include: (1) a method of utilizing a mechanism, such as a safety valve provided in a sealing plate, which detects an internal pressure of a battery and cuts off the current; (2) a method in which a sealing plate is provided with a component composed of a PTC (Positive temperature coefficient) element whose electrical resistance increases in response to the heat generation in the battery, and the current is cut off when the PTC element becomes a nonconductor; and (3) a method in which an insulation layer that is melted in response to heat generation in the battery is used, and when the insulation layer melts, movement of a lithium ion between a positive electrode and a negative electrode is inhibited so that the current is cut off.
Meanwhile, as another method for cutting off a current to suppress heat generation, an electrode provided with a PTC layer has been proposed (for example, refer to Japanese Patent Application Laid-Open (JP-A) No. 2009-176599). A PTC layer indicates, similar to a PTC element, a layer having a function to increase the electrical resistance (direct current resistance) in response to heat generation of a battery. The electrode (at least one of a positive electrode or a negative electrode) according to JP-A No. 2009-176599 is a layered body in which a positive electrode active material layer or a negative electrode active material layer, a PTC layer, and a current collector are layered in this order.
Further, a method in which a PTC conductive material whose electrical resistance (direct current resistance) increases in response to heat generation in the battery is used in an electrode active material has been proposed in JP-A No. H10-241665.